This invention relates to connecting line voltage drop compensating circuits and in particular to the application of a dual input differential amplifier to cancellation of line voltage drops in pulse width modulated power amplifier circuits.
Circuits of the type to which the invention applies generally consist of a power amplifier fed from a power source and feeding output terminals to which a load is connected. Long variable impedance connecting lines connect the amplifier to the output terminals. Such circuits are subject to the adverse effects of the impedance of the long varying impedance connecting lines from the source impedance "seen" at the terminals to the load.
The state-of-the-art approach to eliminating these adverse effects is to have a very high gain feedback loop enclose the connecting line resistances. The magnitude of these resistances as seen by the load is then, by classic theory, divided by the magnitude of the loop gain and reduced in inverse proportion to this value.
The basic disadvantage of this method is that if a very low-source impedance (-.differential.e.sub.o /.differential.i.sub.L)) is desired and connecting line resistances are much higher than the required value, the required "loop gain" must be quite high. Then if the desired impedance must remain low at fairly high frequencies serious feedback loop stability problems easily arise and preclude the effective use of this method.
This situation is particularly serious in the case of modern "pulse width modulated" amplifiers. These use alternating polarity "switched" pulses with pulses of the desired output polarity lengthened and their opposites shortened to produce a required output. The frequency of alternation of these pulses must be much higher than the highest required output-signal frequency, with the undesired high frequencies being removed by a filter at the output.
In this scheme control of high-frequency interference usually requires that the filter be close to its source and since the filter cutoff requirements make it impossible to make it an element of a wide-band, high-gain feedback loop feedback loop gain including the filter is limited.
Accordingly there currently exists the need for a connecting line voltage drop compensator for use in pulse width modulated amplifier and similar circuitry that is not subject to the limitations described. The present invention is directed toward satisfying that need.